Here’s a look at the key characteristics to consider before you buy batteries.
Size & Weight
The more voltage, the more speed. Though the minimum for a highway-capable vehicle is 96 V, some systems go to 300 V or more. The physical size of the battery—volume per volt—will determine your top speed, so you’ll need to figure out how many volts can fit into the space available and whether that voltage will be enough to reach your desired speed.
Look at volume per volt—how bulky should the pack be to give me the speed I want? Then examine watt-hour (Wh) capacity per pound (a.k.a. “energy density”)—how heavy is the pack I need to give me the range I want?
For example, a 96 V pack of 6 V FLA batteries will take up approximately 7.35 cubic feet. A pack of the same voltage (i.e., yielding the same top speed) using 12 V SLA batteries would only take up 2.45 cubic feet. The flooded 6 V pack weighs about 1,072 pounds, while the sealed pack of 12 V units tops out at 350 pounds.
Saving space and shedding pounds may be necessary with smaller vehicles, but consider your range requirements first. The flooded pack has a 23.2 kilowatt-hour capacity compared to only 5.3 kWh for the sealed pack. So while the flooded pack weighs about three times as much and takes up three times as much space, it has almost four and a half times as much range capacity. The sealed pack will require more care in charging and will be almost three times the price for an equivalent amount of kWh, not counting the cost of the battery management system (BMS).
A battery pack is made up of many batteries interconnected to form one power source. The number of interconnects needed to achieve the series voltage will determine the ease of the job and the pack’s overall reliability. Every time you double the number of connections, you quadruple the number of possible failure points.
The type of interconnect used is dictated by the battery terminal style. Clamps are used on round automotive-style terminals, while flat lugs are bolted to flat “L-style” terminals or sometimes directly into the top of the battery. Avoid using lugs over threaded studs sticking up from lead posts—lead is soft and allows the stud to gradually creep upward to relieve the tension of the nut holding the lug. If neglected for too long, this loose connection can melt off the battery terminal or start a fire.
To obtain the desired voltage and amp-hours, batteries can be configured in parallel or in series, or a combination of both.
- In a parallel circuit, positive posts are connected to positive, and negative posts are connected to negative. In this configuration, amp-hour rating is multiplied by the number of batteries and the voltage stays the same as in a single battery.
- In a series circuit (which is most common in an EV), batteries are connected positive to negative. The voltage is multiplied by the number of batteries, and the amp-hour rating stays the same as a single battery.
All batteries should be checked periodically for clean and tight connections. In particular, FLAs require regular fluid checks and, when necessary, their electrolyte levels must be topped off with distilled water. FLA packs also need to be “equalized” to keep all the batteries in balance at the same state of charge.
Rate & Depth of Discharge
Battery life is related to two primary factors: (1) the rate of discharge—how quickly energy is drawn out of the battery (measured in amps); and (2) the depth of discharge—how low the voltage can drop before recharging (measured as a percentage). The less severe the rate or depth of discharge, the longer the battery’s life cycle.
Some batteries can release large amounts of energy more quickly than others. There are also differences in how deeply they can be discharged without causing damage. If either issue is a high priority for your use, be sure to consider rate or depth of discharge when battery shopping.
Balancing & Management Systems
Imbalances among batteries are typical within battery packs—the same make and model can have small differences in voltages. Over time, these imbalances tend to grow, shortening the life of the pack, so regularly rebalancing or “equalizing” the pack is necessary.
When a battery pack is charging, some batteries reach full charge sooner than others. With equalization, the charging continues until the “weaker” ones catch up with the other batteries. The key is not to overcharge the “stronger” batteries, since overcharging can irreversibly damage or destroy a battery.
Some batteries—typically older, “low-tech” FLA batteries—can be balanced by a standard charger. To ensure safe and effective charging, most batteries require a BMS with a sensor/control for each battery that monitors the voltage and controls the charge going into the battery. More sophisticated systems may have circuits for each battery, while simpler versions have a voltage tap running from each battery to a voltmeter. Some require additional wiring and parts, while others are already built into the battery.
Typically, a BMS adds to the cost of the vehicle and the complexity of the wiring. Expect to spend half the cost of the battery pack, and be sure to allow extra time—and patience—for installation. Follow your battery manufacturer’s recommendations for the BMS.
Physical Containment & Ventilation
All batteries must be physically contained, but some require additional measures. Some need to be tightly packed, while others need space between them. Ventilation may be required for carrying away fumes or cooling purposes. Conversely, some batteries may need to be kept warm in cold climates. These packaging considerations affect how many batteries will fit into the vehicle and where they can be placed, as well as the overall cost of the installation.